There is an emerging market for two-way wireless data transceivers to be used with battery-operated, portable host devices such as laptop computers and "personal digital assistants". Such wireless transceivers permit data to be exchanged among host devices in a packet format, consisting of a predetermined number of data bits, as well as other bits which are used in housekeeping functions such as clock recovery and error detection and/or correction. When a packet is sent, the transceiver transmitter operates momentarily, followed by a time period of a much longer duration when the transceiver is either receiving or idle. The current peak drawn when the transmitter is active is much larger than the current drawn when the transceiver is receiving or idle. Power for the transceiver power is supplied from the host device battery, and may pass through an interface connector defined by an industry standard such as the Personal Computer Memory Card International Association, PCMCIA. The trend to lower voltage power distribution in host devices will eventually lead to an increase in the transmitter current required to maintain the same output power.
The voltage at the transceiver may be reduced to an unacceptable level when current is drawn from the host device battery due to power distribution wiring resistance and the equivalent series resistance of the host device battery itself. The resistance, R, of an electrical conductor depends on the resistivity, .rho., the length, I, and the cross-sectional area, A, in the following manner: ##EQU1## Thus, the trend toward higher printed circuit board density will increase the likelihood of higher wiring resistance. The equivalent resistance of the host device battery depends on many factors, including its chemical composition and physical parameters. Nickel cadmium (NiCd) batteries have the lowest internal resistance. Nickel metal hydride (NiMH) batteries are becoming more common in host devices since their energy density is 30% to 50% higher than NiCd, and they are environmentally safer since they do not contain poisonous cadmium; however, they have a higher internal resistance than NiCd. For a given battery type, small cells have a higher internal resistance than large cells.
It is also possible that the magnitude of the current drawn by the transmitter will exceed the limits defined for pins in the interface connector. Typical PCMCIA connectors pins are limited to 300 to 500 mA, depending on the connector design.
It is common for host devices to employ power management to disconnect the battery voltage from peripheral devices when the computer is turned off, or at other times in order to reduce current. Peripheral interfaces, such as the PCMCIA interface, do not provide for a power connection that provides a continuous connection to the battery.
A secondary battery may be safely charged in minimum time by a current having a magnitude C, the ampere-hour rating of the battery. However, overheating can occur as near full charge is reached if this current is delivered continuously. A secondary battery may be charged for several hours at a current of 0.1 C, or continuously at 0.05 C or less. All charge storage devices, including batteries, experience charge leakage at a low rate when idle. The charge lost in this manner must be replaced to maintain full capacity.
Prior-art methods for controlling the flow of charge include commercially-available battery management and remaining energy indicator ICs, and charge controllers. Such ICs are not suitable for the application described since they do not automatically initiate the charging process following disconnect of the load, or may not monitor conditions to prevent overcharging.
Accordingly, there is a need for a charge storage device and a method for controlling the flow of charge such that stored charge is transferred to a load at a predetermined rate when the load is momentarily connected, and replaced by a charging source at a lower rate immediately following disconnect of the load so that the recharging process is likely to be completed while the charging voltage is available.